Compositional Limitations on Rock Types Contributing to Zones of Sealing Phyllosilicate-Rich Fault Gouge
Richard G. Gibson
A common approach to fault-zone seal analysis in siliciclastic strata involves analyzing the lithologic composition of the displaced stratigraphic interval along a fault as a means of predicting the spatial extent of sealing phyllosilicate-rich gouge zones. It is often assumed that only the mudstones within the stratigraphic section contribute to the formation of this phyllosilicate-rich gouge. This study documents composition-dependent contrasts in natural deformational style of sandstones and mudstones that challenge this assumption.
Faulting of phyllosilicate-poor (<20%), porous sandstones results in the formation of deformation bands. In contrast, the deformation of mineralogically less-mature sandstones generates gouge with a fine-grained, phyllosilicate-dominated matrix. Capillary-pressure tests indicate that this phyllosilicate-rich gouge is capable of sealing large hydrocarbon columns, whereas the deformation bands generally do not have small enough pore throats to serve as good seals.
Most mudstones deform by intergranular sliding and other grain-scale mechanisms to form "scaly" gouge typical of sealing "shale smear" fault zones. Other mudstones, however, deform by transgranular fracturing and brecciation, potentially leading to the formation of fluid conduits rather than seals. A transition in deformation mechanism from fracturing to intergranular sliding typically occurs when phyllosilicate content of the original mudstone exceeds 20-30%.
At natural conditions suitable for petroleum preservation, faulting of sandstones and mudstones containing in excess of 20-30% phyllosilicates generally produces phyllosilicate-rich fault gouge. Thus, during fault-seal analysis in siliciclastic sequences, this compositional criterion can be used to distinguish between lithologies that do and do not contribute to the formation of high-quality sealing fault gouge.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California